A) Field of the Invention
The present invention relates to a light emitting device and its manufacture method, and more particularly to a light emitting device equipped with a reflector and its manufacture method.
B) Description of the Related Art
Semiconductor light emitting elements such as light emitting diodes (LED) are used in an information display light source and the like, as compact and high efficiency light sources. Improvements on brightness and efficiency have recently progressed so that application of semiconductor light emitting devices has extended to vehicle illumination and general illumination. A light emitting device is constituted of a semiconductor chip, a container for housing the semiconductor chip, an electronic circuit and the like.
The container has a recess accommodating the components and constituted of a single member or a combination of a plurality of members. The container is made of ceramic, glass, resin, conductor whose surface is covered with an insulating film or the like. A wiring layer for supplying a light emitting element with electric power is formed extending from the recess bottom of the container to the external.
The light emitting element is fixedly bonded to the wiring layer (on an electrode) on the bottom of the recess, for example, at the bottom surface of the light emitting device, with conductive adhesive. The conductive adhesive commonly used is silver (Ag) paste, solder, and eutectic material such as AuSn. The electrode on the top surface is connected to a wiring layer by an Au wire or the like. Two electrodes may be formed on the top surface side, or flip-chip bonding may be performed.
Resin such as silicone resin which contains, if necessary, dispersed fluorescence emission material (fluorescent material), is filled in the recess to seal the light emitting element and form a light emitting device. The fluorescent material absorbs at least a portion of emission from a semiconductor chip and emits wavelength converted light.
Light emitted from the semiconductor chip repeats reflection and absorption at the fluorescent member and the inner surface of the recess of the container, and is finally output to the external. If the material of the container exhibits absorption relative to light irradiated from the semiconductor chip or fluorescent material, light is absorbed in the recess and attenuated. Therefore, luminance of the light emitting device is lowered.
If the material of the container has optical permeability relative to light irradiated from the semiconductor chip or fluorescent member, the light transmits through the inner wall of the recess and leaks to the external other than the opening. Therefore, luminance of the light emitting device is lowered.
It is known that a reflector such as a reflection layer is formed on the recess inner surface of the container. An electrode serving also as a metal reflector is patterned on the recess inner surface in some cases. The electrode serving also as the electrode is not formed on the whole recess inner surface, but is often formed partially in accordance with an electrode pattern for feeding power to the LED chip. For example, patterning on the recess bottom (substrate surface) with an insulating space, patterning on the recess bottom and inner sidewall with an insulating space, or patterning on the recess inner sidewall with an insulating space, or the like, may be employed. The insulating space prevents electric short while power is supplied to the semiconductor chip.
Making a light emitting element container compact and thin has recently advanced rapidly. A space between reflector patterns formed on the recess inner surface of such a compact and thin container is desired to be as narrow as possible to increase a light use efficiency. If this space is made narrower than 0.3 mm, electric short is likely to occur in practice.
The material of the reflection layer is often metal such as Ag, Au, Cu and Al. Ag is suitable because it has a high reflectivity in the visible light range. Since Ag has a high reflectivity of 90% or more in the near infrared to visible light range, lowering luminance of the light emitting device can be suppressed relatively small. For example, a metalized layer is coated on the surface of an insulating board, and Ag is plated on the metalized layer. Alternatively, print ink made of Ag-containing material is coated or patterned by screen printing or the like and then sintered to form an Ag layer.
Ag has a property that it is likely to have chemical change when it contacts chemical substance containing sulfur, halogen or the like. For example, if Ag contacts sulfur contained in atmospheric air, there is a possibility that silver sulfide is formed. As silver sulfide is formed, the surface is discolored to black or gray. Light irradiated from the semiconductor chip or fluorescent member is absorbed in the discolored area so that luminance of the light emitting device is lowered. Ag has low adhesion to resin such as silicone. Lower luminance and defects due to interface stripping are likely occur during a long term use. A reflectivity in the ultraviolet range of 400 nm or shorter is 0 to 90% lower than that in the near infrared to visible light range, and is insufficient for use with the light emitting device.
The reflector is not required to have mirror reflection. It is sufficient if the reflector reflects light irradiated from LED and outputs the light to the external. Therefore, the reflector may be irregular or scattering reflector. A reflector with irregular reflection may be ceramics. White ceramics have a flat reflectivity in the visible light range. A reflection ring forming a reflection surface around LED is known. A ceramic substrate serving also as a reflector has been used as the substrate for mounting a semiconductor light emitting element.
JP-A-HEI-9-293904 proposes to plate gold (Au) on a connection terminal formed on the rear surface of an LED package to be connected to a wiring substrate and to plate silver (Ag) on a power feeding wiring formed on the top surface of the LED package to be connected to LED. If both the terminals are plated with Au, reflectivity at wavelengths shorter than 600 nm lowers. By plating the power feeding wiring on the top surface of the package with Ag, a high reflectivity is obtained even at a wavelength of 600 nm or shorter.
JP-A-2005-179147 proposes an LED ceramic substrate having a high reflectivity layer containing alumina, silica and magnesium. It teaches that it is preferable to set a silica content to 0.1 to 1 mass %, and a magnesium content to 0.01 to 0.5 mass %. It also teaches that as the silica content becomes lower than 0.1 mass %, a reflectivity lowers sharply, a peel strength rises sharply as the silica content becomes about 1 mass % or lower, a peel strength lowers sharply as a magnesium content becomes lower than 0.01 mass %. It describes that as a cover layer with a small silica content is formed on an area outside an LED mount area, a peel strength improves when wirings are formed. It also teaches that it is preferable to prevent inner pores from becoming large, in order to prevent the strength from being lowered. A measured bulk density and a measured apparent density take the same value. A reflectivity of disclosed ceramics is about 80% at most in the visible light range at a wavelength of 470 nm.